Ferrite process



United States Patent 3,274,110 FERRITE PROCESS Allan Getto, Poughkeepsie, N.Y., assignor to International Business Machines Corporation, New York, N.Y., a corporation of New York No Drawing. Filed Dec. 21, 1962, Ser. No. 246,340 2 Claims. (Cl. 25262.5)

This invention relates to ferromagnetic materials of the spinel type generally known as ferrites, and, in particular, to an improved process for producing bodies of such materials so as to exhibit maximum squareness of the hysteresis characteristics.

Ferrospinel bodies are employed as magnetic memory elements and as pulse transfer elements in computer and other data process machines. The characteristic shape of the hysteresis loop for these bodies is of particular importance, especially where the ferrospinel body is employed as a coincident current memory device which, to a large degree, depends on the ferrospinel body exhibiting a response excitation characteristic of a substantially square hysteresis loop type. For, in a coincident current memory device, two current pulses, each of which alone is of insufficient intensity to produce switching, are applied to the device to switch the body from one of its remanent states of magnetization to the other. Where maximum squareness is not realized in the device, partial switching may occur with only one pulse to the point that unselected signals are produced which are diflicult to distinguish from selected signals. Memory devices of this type are well-known in the art.

Procedures for producing ferrospinel ferrite bodies that exhibit a response excitation of a substantially square hysteresis loop type are well-known in the art. These ferrites are generally synthesized by mixing selected constituent metallic oxide in predetermined proportions, processing the mixture to a ferrite powder by standard ceramic methods, pressing the powder into rigid shape and, thereafter, sintering the pressed material at a high temperature. The treatment causes the constituents to react and diffuse on an atomic scale to form a ferrospinel crystal structure within the pressed body of material and provides it with two stable states and a response excitation characteristic of the square hysteresis loop type.

In making these ferrites problems arise in maintaining uniformity in the magnetic characteristics between the ferrite bodies resulting from a continuous process. Owing to the many variables which affect the process there is a great opportunity for variance between the magnetic characteristics in the ferrites and, in :many instances, this variance is sufiicient to deteriorate the squareness characteristic to the point that the material is not useful as a memory element. It has been an object of considerable research, therefore, to more fully understand what variables affect the processing of square loop ferrites and to bring these under control.

Surprisingly, it has been discovered that water vapor acts as a catalyst when admitted in a limited amount during the sintering treatment of a ferrite containing a trivalent cation and at least one divalent cation. This catalytic action is of a most beneficial nature: it assures maximum squareness in the ferrite, provides uniformity in the magnetic characteristics in the ferrites resulting from the continuous process and greatly increases the number of ferrites that are useful as memory elements from the fabrication procedure. By admitting water vapor into the sintering ambient in a limited amount during the fabrication process, a greater degree of control is provided in the fabrication of ferrites than heretofore available in the art.

3,2 74 ,1 l0 Patented Sept. 20, 1966 Accordingly it is a primary object of this invention to provide an improved process for making square loop ferrite bodies.

It is a further object of this invention to provide an improved process for maintaining uniformity in the magnetic characteristics of ferrite bodies produced by a continuous process.

It is still a further object of this invention to provide a commercial feasible process for increasing the number of useful square loop ferrite bodies derived from a continuous fabrication procedure.

Generally speaking, this invention provides a method for sintering ferrite bodies which employs a limited amount of water vapor in the ambient during the treatment. It has been found that the water vapor is most effective during the oxygen absorption phase of the sintering treatment. It appears to have some influence on the kinetics of the oxygen pick-up and it is most beneficial when utilized during this phase of the treatment. From what has been determined, the water vapor provides a beneficial catalytic reaction to the ferrite formation reactions which assures maximum squareness and enhances the magnetic characteristics of the material. Those chemical reactions which occur during the oxygen absorption phase of the treatment are recognized as being of particular importance to the fabrication of a square loop ferrite, and, it appears that the water vapor somehow facilitates the reaction mechanisms during this phase when present in a limited amount in the ambient. The role of the water vapor in the oxidation of a ferrite is found to be an extremely important item, but the development of the general principles which explain its function are not presently known.

It may be added that water vapor atmospheres have been used previously in the fabrication of ferrite bodies but these atmospheres furnished a completely different type of ambient to the ferrite and, in turn, performed quite a different role in the fabrication of the ferrite. In those instances the water vapor was generally employed as a neutral atmosphere or what has come to be known as a non-destructive atmosphere and used in amounts which are not conducive to oxygen absorption-an important requirement of the present invention.

The amount of water vapor that can be beneficially tolerated in the ambient during the sintering treatment is critical. Where air is the other essential constituent of the ambient, adverse effects are found when the water vapor composes more than 3% by volume of the ambient while complete deterioration is experienced with more than 5% by volume. As to the threshold level required, it is preferable to employ air-water vapor .ambients that contain at least /2 by volume of Water vapor, although lower amounts may be used if desired.

Now, more particularly, the sintering treatment to bring about the final thermal reaction and crystallization includes a firing and annealing step. The firing is performed at a temperature at which the spinel phase is stable and at which crystal grain growth continues, thereby permitting the cations and the anions of the mixed constituents to diffuse and form the spinel structure. The annealing step involves a brief cooling from the firing temperature, which usually (the firing) is in the range from 1100 C. to 1500 C., to a temperature below the firing temperature but which is sufiiciently high to maintain the oxygen absorption and chemical formation reactions. It is during the second phase of the sintering treatment-the coolingthat the utilization of the water vapor in limited amounts is most effective. From what has been found the ambient surrounding the ferrites during the firing phase is not critical, provided it is not a reducing atmosphere, but the ambient that surrounds the ferrite during the annealing step, that is cooling the ferrite to a temperature lower than the firing temperature but one which is sufiiciently high to maintain oxidation and chemical formation reactions, is most critical. During the cooling step it is essential that the ambient permit oxygen absorption and, as heretofore indicated, the water vapor assists in this requirement.

To more fully explain the present invention, specific examples of the process are given which are not intended by way of limitation but are only presented for purposes of illustration and explanation.

Prior to undergoing the sintering treatment of the present invention, conventional ceramic processing methods are used to convert metal oxide raw materials into ferrites having desired magnetic characteristics. The procedure is to start with oxide raw materials which are substantially 4 bient about the ferrite during the firing step need not in clude the air-water vapor mixture but in many sintering systems presently available it may be more economical to include it rather than to exclude it.

The amount of water vapor in the air-water vapor mixture varies from about /2 to 3 by volume of the total amlbient surrounding the ferrite. The upper limit of the Water vapor utilized in the ambient as heretofore stated is most critical in that the square loop characteristics deteriorate if more than 5% water vapor is utilized. The beneficial effects of the water vapor are illustrated by the data in the table below for an air-water vapor ambient. In the table uV is the undisturbed one signal, rV the read disturbed one signal, WV the write disturbed zero signal, T the switching time, and rV /wV the one to zero ratio which is a standard definition for squareness in the art:

Table Sintering Magnetic Parameters Water Percent Comp. Temp, H O Firing Cycle C. by vol. Annealing 1N rV1 [Lvz '1:

Temp, (mv.) (mv.) (mv.) 1sec. rVilwV Temp. Time 0.

stagnant Air 1, 200 8 920 40 38 9. 2 1. 00 4. 1 3. 0 1, 200 8 920 44 41. 5 9. 9 1.02 4. 5 37 6. 0 1, 200 8 920 56 22 48 96 55 47 10. 5 1, 200 6% 920 60 13 54 8 25 62 21. 5 1, 200 6% 920 56 12 50 75 25 82 50. 0 1, 200 6% 920 54 15 47 31 100 100. 0 1, 200 6% 920 18 12 10 1. 2

pure, and have as fine a particle size as are commercially obtainable. These oxide raw materials are then mixed with a medium, usually water, and milled in a steel ball mill. Although the ball mill is commonly used, other mixing devices such as colloid mills and attritors may be used. After milling the homogeneous oxide mixture is oven dried, and the resulting cake after pulverization is presintered, which consists of heat treating the oxide mixture at a temperature somewhat lower than the final firing temperature. All or part of the oxide mixture may be presintered for it has been shown that the presintering is helpful in controlling shrinkage in the final shape, and influences grain parameters, size and shape, and homogeneity; however, the presintering step may be completely eliminated in certain manufacturing processes. After presintering, or if no presintering is used, after calcining, the oxide mixture is comminuted to a particle size that is ceramically workable, and organic binders are added to serve as a binder and particle lubricant. Conventional means such as dry presses and extruding are then used to press the oxide mixture into desired shapes. The shaped metal oxide is then in the final stages before it undergoes the process of the present invention.

With a ferrite including a trivalent cation and a divalent cation species such as, for example, 40% Fe O 55% MnCO and 5% CuO, the calcining is done at a temperature from 600 C. to 1000 C. for approximately at a time interval ranging from to 180 minutes. In the instance cited, the binder is 3% polyvinyl alcohol and the lubricant by weight dibutyl phthalate. The pressing is done to a density between about 2.5 to 4.0 grams per cubic centimeter.

Following the conventional ceramic procedures the Cu-Mn ferrite is fired at 1250 C. for about 10 minutes (the time and temperature is a function of the composition and may vary from 1100 C. to 1500 C. for periods of 2 minutes to 4 hours). After the firing step, the ferrite is annealed in an ambient containing air and water vapor in amounts from about /2% to 3% by volume. The annealing step entails cooling the ferrite to a temperature of about 900 C. which for the Cu-Mn ferrite requires in about 10 to 15 minutes. .As previously indicated the am- Reference to the table indicates that in the ambients containing more than 5% water vapor, the one-to-zero disturbed signal ratio is drastically affected. Note that at 3% water vapor the rV /wV is 4.5 while at 6% water vapor the ratio is 0.55. Ferrites from the latter treatment give extremely poor responses when used as coincident memory devices. The situation is further aggravated as the amount of Water vapor is increased in the ambient.

With a threshold level of water vapor in the air ambient surrounding the ferrite during the sintering treatment, a catalytic reaction is produced which assures maximizing the square loop characteristics and increases the yield of ferrites from a continuous process treatment of the material. As stated elsewhere, the exact nature of the catalytic effect of the water vapor has not been determined and various explanations may be offered to explain the effect of the water vapor. It may be that the water vapor produces surface migration of the ions and promotes the solid-state reactions which are so necessary in the fabrication of the square loop ferrite. Or, it may be that the water vapor facilitates nucleation, promotes surface diffusion and expands the reaction sites. Although the mechanism responsible is undetermined, it is conclusively shown to be most beneficial in the sintering treatment.

The invention is applicable to any ferrite system in which oxygen is absorbed during the annealing step. Several examples may be cited of typical three cation and two cation ferrite systems which benefit from the present invention, such as: Mn-Zn-Fe, Ni-Zn-Fe, Mg-Mn-Fe, MnFe, Ni-Fe, Zn-Fe, Cu-Fe and Mg-Fe. In these systems, the firing is performed at temperatures between 1100 C. to 15 00 C. while the temperature to which these ferrites are annealed is necessarily dependent upon the particular ferrite composition in question, it is generally above 600 C.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein Without departing from the spirit and scope of the invention.

What is claimed is: 1. The process of sintering a rectangular hysteresis loop ferrite structure of the ferrospinel system to enhance rectangularity of the hysteresis characteristic by the steps of: preparing an intimate mixture of manganese, copper and ferric oxides wherein on a mol basis the major proportion of said mixture is manganese oxide, the minor proportion of said mixture is copper oxide and the proportion of iron oxide is therebetween;

calcining said mixture at a temperature in the range between 600 C. to 1000 C.;

firing said mixture at a temperature in the range between 1100 C. to 1500 C. to produce solid state reactions within said mixture to provide same with a spinel crystal structure; and,

annealing said ferrite mixture by cooling said fired mixture from said firing temperature to a temperature above 600 C. which is sufliciently high to enhance oxygen absorption, said cooling being carried out in a controlled oxidizing ambient consisting essentially of air that includes from /z% to 5% by volume of water vapor.

2. The process of sintering a rectangular hysteresis loop ferrite structure of the copper-manganese ferrospinel system to enhance irectangularity of the hysteresis characteristic by the steps of:

preparing an intimate mixture of metallic oxides including about 55 mol percent of manganese oxide,

about 5 mol percent copper oxide, and the balance ferric oxide;

calcining said mixture at a temperature in the range between 600 C. to 1000 C.;

firing said mixture at a temperature in the range between 1100 C. to 1500 C. to produce solid state reactions within said mixture to provide said mixture with a spinel crystal structure; and

annealing said ferrite mixture by cooling said fired mixture from said firing temperature to a temperature which is sufliciently high to enhance oxygen absorption, said cooling being carried out in a controlled oxidizing ambient consisting essentially of air that includes from /2 to 5% by volume of Water vapor.

References Cited by the Examiner UNITED STATES PATENTS 2,818,387 12/1957 Beck et al. 2,988,508 6/1961 Geldermans et al. 2,994,522 8/1961 Albers-Schoenberg. 3,031,407 4/1962 Di Marco. 3,100,194 8/1963 Van der Burgt et al.

TOBIAS E. LEVOW, Primary Examiner.

MAURICE A. BRINDISI, S. R. BRESCH, R. D.

EDMONDS, Assistant Examiners. 

1. THE PROCESS OF SINTERING A RECTANGULAR HYSTERESIS LOOP FERRITE STRUCTURE OF THE FERROSPINEL SYSTEM TO ENHANCE RECTANGULARITY OF THE HYSTERESIS CHARACTERISTIC BY THE STEPS OF: PREPARING AN INTIMATE MIXTURE OF MANGANESE, COPPER AND FERRIC OXIDES WHEREIN ON A MOL BASIS THE MAJOR PROPORTION OF SAID MIXTURE IS MANGANESE OXIDE, THE MINOR PROPORTION OF SAID MIXTURE IS COPPER OXIDE AND THE PROPORTION OF IRON OXIDE IS THEREBETWEEN; CALCINING SAID MIXTURE AT A TEMPERATURE IN THE RANGE BETWEEN 600*C. TO 1000*C.; FIRING SAID MIXTURE AT A TEMPERATURE IN THE RANGE BETWEEN 1100*C. TO 1500*C. TO PRODUCE SOLID STATE REACTIONS WITHIN SAID MIXTURE TO PROVIDE SAME WITH A SPINEL CRYSTAL STRUCTURE; AND, ANEEALING SAID FERRITE MIXTURE BY COOLING SAID FIRED MIXTURE FROM SAID FIRING TEMPERATURE TO A TEMPERATURE ABOVE 600*C. WHICH IS SUFFICIENTLY HIGH TO ENHANCE OXYGEN ABSORPTION, SAID COOLING BEING CARRIED OUT IN A CONTROLLED OXIDIZING AMBIENT CONSISTING ESSENTIALLY OF AIR THAT INCLUDES FROM 1/2% TO 5% BY VOLUME OF WATER VAPOR. 